Both dry and liquid developers are used for the development of latent electrostatic charge images. Dry developers cause some problems. Dusting is one of them. Another is the difficulty of controlling distribution of the dry toner powder on the latent image. A further problem is that the rather large particle size of the ground toner particles limits the fineness of the image resolution.
Liquid developers as described, for example, in Metcalfe et al, U.S. Pat. No. 2,907,674, and as widely used in office copying machines that copy documents on zinc oxide coated paper, avoid or reduce certain of the problems of dry developers.
Liquid electrographic developers are a dispersion of toner particles, usually of a pigment and a binder resin, in an electrically-insulating carrier liquid such as a volatile hydrocarbon fraction. The binder resin serves to bind the pigment to the copy paper on which the latent electrostatic image is developed. The developer can also contain a stabilizer additive to maintain a stable dispersion of the toner particles, although in practice many so-called "stabilized" liquid developers prove unstable after a period of time, especially when the initial toner concentrate is diluted with carrier liquid to form a working strength developer. Liquid developers also often contain a charge control agent to maintain a constant charge level.
Stabilizers for liquid developers are described, for example, in York, U.S. Pat. No. 2,899,335 and U.K. Pat. No. 1,065,796. Charge control agents, including certain quaternary ammonium salts, are disclosed in Beyer, U.S. Pat. No. 3,417,019 and Tsuneda, U.S. Pat. No. 3,977,983 for liquid developers and in Jadwin et al U.S. Pat. No. 3,893,935 for dry developers.
For reasons not fully understood, most binder resins develop a positive charge when dispersed in an insulating carrier liquid. Negatively charged developers are also needed, however. For instance, some electrographic microfilm reader-printers employ both a negatively charged and a positively charged liquid developer so that the user can switch from one to the other if he wishes to reverse the printing of the image.
Unfortunately, even the few binder resins that develop a negative charge have drawbacks. For instance, halogenated polymers offer promise as negatively charged binder resins but their use has been impeded by the difficulty of dispersing them stably in the usual electrically insulating carrier liquids.